Influence Of Coulomb Collisions Research Articles

Influence of the beam-beam collisions on the energetic particle distribution in large helical device

The influence of Coulomb collisions between energetic particles in a neutral beam injection (NBI)-heated plasma is numerically investigated utilizing the improved collision operator for the orbit-following type of Monte Carlo codes. This nonlinear collision operator enables the collisional effect by non-Maxwellian plasmas including energetic particles to be taken into account. We have implemented it into GNET, one of the orbit-following Monte Carlo codes solving drift kinetic equation in the 5D phase-space, and performed simulations for tangentially injected NBI cases (beam energy: 180 keV), taking account of beam–beam Coulomb collisions. It is found that the beam–beam collisions cause energetic particle orbits to transition from passing to trapped and enhance the collisional transport, which results in the deterioration of the energetic particle confinement.

Inward pinch of high-Z impurity in a rotating tokamak plasma: effects of atomic processes, radial electric field and Coulomb collisions

The transport of high-Z impurity in a toroidally rotating tokamak plasma is investigated analytically and numerically. It is shown that the inward pinch is driven by the atomic processes of ionization/recombination along the particle orbit both in co- and ctr-rotating plasmas. This inward pinch is enhanced by the radial electric field. It is derived that the negative and positive radial electric fields cause the inward pinch and the outward movement (unpinch) of the high-Z impurity, respectively, under the influence of Coulomb collisions with the rotating background plasma. In the ctr-rotation case, the inward pinch becomes significant with increasing toroidal rotation velocity, because the directions of both pinches are inwards. On the other hand, in the co-rotation case, these pinches have opposite directions. Therefore, the unpinch due to the positive radial electric field is decreased by the inward pinch due to the atomic processes. These tendencies are consistent with the tungsten accumulation observed in the JT-60U rotation scan experiment.

Driven magnetic reconnection near the Dreicer limit

The influence of Coulomb collisions on the dynamics of driven magnetic reconnection in geometry mimicking the Magnetic Reconnection eXperiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] is investigated using two-dimensional (2D) fully kinetic simulations with a Monte Carlo treatment of the collision operator. For values of collisionality typical of MRX, the reconnection mechanism is shown to be a combination of collisionless effects, represented by off-diagonal terms in the electron stress tensor, and collisional momentum exchange between electrons and ions. The ratio of the reconnection electric field ER to the critical runaway field Ecrit provides a convenient measure of the relative importance of these two mechanisms. The structure of electron-scale reconnection layers in the presence of collisions is investigated in light of the previously reported [S. Dorfman et al., Phys. Plasmas 15, 102107 (2008)] discrepancy in the width of the electron reconnection layers between collisionless simulations and experimental observations. It is demonstrated that the width of the layer increases in the presence of collisions, but does not substantially deviate from its collisionless values, given by the electron crossing orbit width, unless ER⪡Ecrit. Comparison with MRX observations demonstrates that the layer width in 2D simulations with Coulomb collisions is substantially smaller than the value observed in the low-density experiments with ER≲Ecrit, indicating that physical mechanisms beyond those included in the simulations control the structure of the electron layers in these experiments.

Influence of Coulomb collisions on the structure of reconnection layers

The influence of Coulomb collisions on the structure of reconnection layers is examined in neutral sheet geometry using fully kinetic simulations with a Monte Carlo treatment of the Fokker–Planck operator. The algorithm is first carefully benchmarked against key predictions from transport theory, including the parallel and perpendicular resistivities as well as the thermal force. The results demonstrate that the collisionality is accurately specified, thus allowing the initial Lundquist number to be chosen as desired. For modest Lundquist numbers S≲1000, the classic Sweet–Parker solution is recovered. Furthermore, a distinct transition to a faster kinetic regime is observed when the thickness of the resistive layer δSP falls below the ion inertial length di. For higher Lundquist numbers S≳1000, plasmoids (secondary islands) are observed within the elongated resistive layers. These plasmoids give rise to a measurable increase in the reconnection rate and for certain cases induce a transition to kinetic regimes sooner than expected from the δSP≈di condition. During this transition, the reconnection electric field exceeds the runaway limit, leading to electron scale current layers in which the nonideal electric field is supported predominantly by off-diagonal components in the electron pressure tensor, along with a residual contribution from electron-ion momentum exchange. These weakly collisional electron layers are also unstable to the formation of new plasmoids.

Influence of Coulomb collisions on isotopic and elemental fractionation in the solar wind acceleration process

In view of new observational evidence from isotope spectrometers on WIND, SOHO (Solar Heliospheric Observatory), and ACE (Advanced Composition Explorer), we explore the efficiency of isotope fractionation processes in the inner corona. We reinvestigate the role of Coulomb collisions in the acceleration of minor ions using a multifluid model. To model the main gas, we study stationary solutions for the continuity and momentum equations of electrons, protons, and alpha particles. As a closure of the system of equations, we prescribe expansion geometry and temperature profiles based on observations. The behavior of minor ions, which are treated as test particles, depends in a complicated manner on their mass and on their charge, structured by the interplay of acceleration, gravity, pressure gradient, electromagnetic fields, Coulomb drag, and thermal diffusion. We compare the fractionation effects in different solar wind regimes: In our model high‐speed solar wind emanating from polar coronal holes, Coulomb friction practically equalizes the velocities of all species, and no substantial fractionation takes place. In the case of a rapidly expanding magnetic field geometry, for example, in the vicinity of a coronal streamer, the proton flux and thus the Coulomb friction on minor ions is reduced, leading to depletion of heavy species in the solar wind. The model also predicts a substantial depletion of alpha particles relative to protons in the heliospheric current sheet, consistent with observations. In such a situation, heavy elements are depleted in the solar wind relative to protons as well, but the effect is strongest for alpha particles. Isotopic fractionation of helium of the order of 30% is possible, while the isotope effect on heavier elements amounts at most to a few percent per mass unit.

Wave Action in a Plasma with Coulomb Collisions

The influence of Coulomb collisions on the wave propagation inplasmas is considered. It is demonstrated that the nonlinear couplingof the nonresonant wave with electric field fluctuations resonant withthe plasma particles as well as contribution of the systemnonstationarity induced by the collisions is crucial to obtain thecorrect conservation equation for the wave occupation number. Incontrast to the occupation number, the canonical wave energy changesbecause of the nonstationarity of the system. Since the definition ofthe wave energy is not necessarily unique and depends on separationinto background and wave subsystems, the result for the change of thecanonical wave energy is not universal. An alternative way ofseparation into background and wave subsystems isdiscussed.

On the influence of Coulomb collisions on the electron energy distribution function of surface wave produced argon plasmas

The effect of Coulomb collisions on the electron energy distribution function (EEDF) in surface-wave produced argon plasmas in the RF and microwave regime is studied via solution of the Boltzmann equation, including a Fokker-Planck term, in the well known Lorentz approximation. The maintenance field strength is determined self-consistently by simultaneously solving a particle balance equation subjected to a boundary condition, which accounts for the sheath formation at the wall. The modifications to the EEDF and some derived parameters introduced by the Coulomb collisions will be discussed. The theoretical predictions are compared to experimental results, which have been obtained by a Langmuir probe technique on a surface-wave produced argon discharge at 60 MHz.

Radial evolution of solar wind thermal electron distributions due to expansion and collisions

ISEE 3 electron observations near 1 AU show that the solar wind thermal electron temperature anisotropy, T∥/T⊥ is typically 1.0 to 1.5, with densest distributions most nearly isotropic, but is sometimes much higher when density is low. For a small observational subset characterized by high density and low bulk speed, T⊥ can exceed T∥. Based on these and other observations, we propose a simple model for radial evolution of thermal electrons in a structureless solar wind under the influence of Coulomb collisions and geometric expansion in a spiral interplanetary magnetic field (IMF). We use the Chew‐Goldberger‐Low double adiabatic invariants, modified by a collisional relaxation term, to develop coupled differential equations for dT∥/dr and dT⊥/dr which can be numerically integrated from an assumed isotropic starting point near the Sun. The model, which satisfactorily explains the 1 AU observations, shows that the evolution of electron temperature and T∥/T⊥ is controlled by plasma density, flow speed, and initial temperature, with density the most important factor. Highest density plasmas cool most rapidly and remain nearly isotropic, while lower density plasmas can develop large thermal anisotropies due to their low collision frequencies and thus can maintain a higher total temperature. Convection speed influences the radial evolution by (1) dictating the IMF spiral, and (2) controlling the time available for collisional relaxation. The model makes predictions for radial distances other than 1 AU and for high solar latitudes. In the inner solar system, where the IMF is nearly radial, the prevailing T∥/T⊥ should be greater than at 1 AU. At greater radial distances in the solar equatorial plane, the increasing inclination of the IMF to the radial direction plus the cumulative effects of collisional relaxation reduce the anisotropy; eventually T⊥ consistently exceeds T∥. At higher solar latitudes the more nearly radial IMF produces a higher anisotropy at all radial distances than at the equator, and the total temperature decreases more slowly with increasing heliocentric distance. We expect that the Ulysses mission, which will return the first full electron distributions from beyond 1 AU and at high heliographic latitudes, will allow tests of these predictions.

Coulomb losses and the nuclear composition of the solar flare accelerated particles

The influence of Coulomb collisions in two-component plasma on the nuclear composition and the charge-state of accelerated particles is investigated. The main characteristics are the location and value of the two loss maxima. It is shown that the maximum of energy losses on the electron component of plasma for flares is a high energy threshold which prevents the penetration of a large particle flux into the range ≳ 10 MeV nucl−1. The low-energy range up to the maximum is considered in detail. At preliminary or initial stage of acceleration the nuclear composition of accelerated particles is strongly dependent on their energy losses on the proton component of plasma which prevails at low energies. The conditions under which the equilibrium charge is reached are investigated.

Influence of Coulomb collisions on quasilinear relaxation of fast ions in a two-component Tokamak

The influence of Coulomb collisions on quasilinear relaxation is theoretically studied. It is assumed that the beam moves along a toroidal magnetic field, and the quasilinear relaxation is caused by the Alfven waves excited by the beam ions due to the beam-Alfven instability, considered previously by Mikhailovskii (1975). It is shown that Coulomb collisions should be taken into account in the analysis of the quasilinear relaxation, if the duration time of beam injection is comparable or exceeds the typical period of such collisions. The efficiency of a two-component Tokamak reactor is determined, the quasilinear and the Coulomb effects being taken into account. It is shown that when calculating the energy balance of the two-component Tokamak reactor, it is necessary to take into account the beam-Alfven instability.

Influence of coulomb collisions on electrostatic waves in the ionosphere

The collisional damping rate of electrostatic LHR waves is calculated. At an altitude of 1000 km, the damping rate is typically a few sec −1. In the absence of an instability the LHR waves should therefore damp out in a time of the order of a second. Horita and Watanabe attribute the LHR noise bands to a Landau instability excited by high energy (~10 keV) electron streams. The instability growth rate depends on the thermal spread in the stream, but can exceed the collisional damping rate if this thermal spread is not too large. One effect of the collisional damping is to shift the lower cutoff of the noise bands up above the LHR frequency, the extent of the shift being dependent on the thermal spread in the electron stream.